(i) Field of the Invention
The present invention relates to a camera lens positioning device for executing a variable focus mechanism of camera such as an autofocusing mechanism, automatic macro mechanism, electrical macro mechanism or zooming mechanism, more specifically to a camera lens positioning device using a shape memory alloy and a camera using the positioning device.
(ii) Description of the Related Art
To take a better photograph, a depth of field as well as a point of focus must be considered. Factors which determine the depth of field are a permissible maximum confusion circle and a resolution. In general, even with imaging devices of the same size, the maximum confusion circle becomes smaller and the range of the depth of field becomes narrower as the resolution becomes higher. For example, even in the field of camera-equipped cellular phones, the resolution has been increasing in recent years (to 2M or 3M, for example), and the range of the depth of field has been becoming increasingly narrower. Therefore, in particular, when the resolution is increased, the center of the depth of field must be determined at a more appropriate position.
The center of the depth of field is determined by the position of a camera lens, as in the case of the point of focus. Thus, for example, to take a better photograph of a closer subject or farther subject, it is necessary to determine the position of the camera lens as highly accurately as possible. Not only for general digital cameras but also for small-sized cameras to be used in small devices such as cellular phones, it is desired that positioning of a camera lens can be carried out with higher accuracy, at low cost, with low electric power and a simple structure and in a small space.
Meanwhile, among conventional small-sized cameras used in cellular phones and the like, there are cameras having a function called a “macro” mode for positioning a camera lens. This “macro” mode function is used to take a photograph of a closer subject. However, execution of the “macro” mode requires manual operation. This operation is obviously cumbersome.
In contrast, in the field of relatively large-sized cameras, an autofocusing function which requires no manual operation is commonly used. However, in this case, as an actuator for moving a lens, such a device as a direct-current motor (DCM) or a stepping motor (STM) is required. Of these DCMs and STMs, some have a diameter of larger than 4 mm and require a relatively large space for installation. Accordingly, when the DCM or STM is used, it is difficult to reduce the size of the device, and these are not suitable for small devices such as cellular phones. Further, the DCM requires gears and a sensor for moving a lens, while the STM requires driving devices such as a lead screw and gears, an electric circuit and a sensor for moving a lens. Thus, they also have problems of high power consumption, complicated structure, high frequency of occurrence of failures and high costs due to mechanical driving.
To move a camera lens automatically, a technique using a shape memory alloy without using the DCM or STM has been developed. The shape memory alloy is favorably used in such small-sized cameras as used in cellular phones since it does not require such devices as gears. For example, in Japanese Patent Application Laid-Open No. 137155/2000, a device is disclosed that moves the position of a camera lens through contact between two plate-shaped shape memory alloys and a portion of the camera lens disposed between the shape memory alloys by allowing the plate-shaped shape memory alloys to undergo deformation according to a change in temperature. However, the disclosed device is a device which merely moves the position of a camera lens according to a change in temperature and has nothing to do with a technique for positioning a camera lens freely. Further, Japanese Patent Application Laid-Open No. 230457/1994 discloses a diaphragm control device that moves a diaphragm member according to a change in temperature by utilizing the action of two fine-line shape memory alloys disposed at the left and right side of the diaphragm member of a camera. However, this device is a device that moves a diaphragm member according to a change in temperature and has nothing to do with a technique for positioning a camera lens freely, as in the case of the above conventional device.
As for a technique for positioning a camera lens, relatively large-sized cameras generally use a multipoint or multistep focusing technique that allows a camera lens to be positioned at multiple positions. According to this technique, a camera lens can be positioned more accurately and the focus of the camera lens can be adjusted more accurately. However, since cameras using this technique generally use the above stepping motor or DC motor as an actuator for moving a camera lens, the same problems as described above occur.